1. Field of the Invention
This invention relates to a steel cord for the reinforcement of rubber articles, and more particularly to a steel cord usable as a reinforcing member in a belt layer for tuck and bus radial tire (TBR).
2. Description of Related Art
As a steel cord for the reinforcement of a belt in TBR, there has hitherto been used a steel cord of three-layer a structure consisting of a core formed by twisting plural steel filaments and two sheaths formed by twisting steel filaments around the core in two layers. Recently, to reduce the weight and simplify the structure in the belt of TBR, there is widely been used a steel cord of two-layer structure consisting of a core and a single sheath.
Among steel cords of two-layer structure, a steel cord having a core wherein plural steel filaments are arranged in line without twisting is known to have the following merits.
(1) The steel cord can be produced at a single twisting step, so that the economic situation is excellent.
(2) Such steel cords are arranged side by side in a belt layer of a tire to extend in a direction of a line connecting the centers of the core filaments in the cord (hereinafter referred to as a core parallel direction) within a plane of the belt layer. As a result the tire having an excellent steering stability is obtained without damaging ride comfort and the like. Also, the thickness of the belt layer can be thinned, so that tire weight can be reduced.
For example, there are disclosed the following techniques with respect to steel cords for the reinforcement of rubber article having a two-layer structure consisting of a core formed by arranging plural core filaments (M filaments) in parallel to each other without twisting and a single sheath formed by twisting plural sheath filaments (N filaments) around the core (hereinafter referred to as M parallel+N structure).
In JP-A-9-158065 is disclosed a steel cord of M parallel+N structure consisting of a core formed by arranging plural core filaments side by side without twisting and a sheath formed by circumscribing plural sheath filaments with the core filaments and twisting the sheath filaments around the core, and having an elliptical shape at its section.
In JP-A-9-156314 is disclosed a steel cord of 2 parallel+N structure (N=5-8) consisting of a core formed by arranging two core filaments of same diameter side by side without twisting and a sheath formed by helically winding 5-8 sheath filaments, each having a diameter corresponding to 0.8-1.2 times the diameter of the core filament, around the core close thereto at a pitch corresponding to 40-60 times the diameter of the sheath filament while forming a gap between the sheath filaments, and having substantially an elliptical shape at its sectional profile.
However, the conventional steel cords of M parallel+N structure have the following problems.
(1) Since the difference in load bearing between the core filament and the sheath filament is large, the efficiency of developing strength and durability are poor.
(2) The core filaments are easy to cross to each other.
(3) Since an internal distorsion remains between the core and the sheath, when a rubberized sheet containing a plurality of such steel cords arranged side by side is cut, it is easy to cause warping at a cut end portion of the sheet. Therefore, the handling of the cut sheet is poor in the production of the tire.